Add Async to ObservableCollections

[thetableman]

This PR adds the ability for an ObservableCollection object to call an awaitable as per discussion #1384.

@falkoschindler I wasn't able to find a way for a ui element to be run within the called awaitable using a parent slot, I also wasn't sure if you wanted the arguments parameter of handle_event to be optional hence why I passed a NoneType.

[falkoschindler]

Thanks a lot for this PR, @thetableman!
In 1738078 I reviewed and improve the code a little (had to merge first because I couldn't push onto your PR). I basically fix some mypy issues, added a sender to the ObservableChangeEventArguments and made sure to actually use these arguments.

[falkoschindler]

By the way, can be tested with this code:

from nicegui import events, observables, ui


async def handle_change(e: events.ObservableChangeEventArguments) -> None:
    await asyncio.sleep(0.1)
    with column:
        ui.label(f'Hello {data["name"]}!')
        print(e.sender)

data = observables.ObservableDict({
    'name': 'World',
}, handle_change)

ui.button('Click me', on_click=lambda: data.update(name=data['name'] + '!'))

column = ui.column()